Serum-free conditioned media from several human tumor cell lines (e.g. rhabdomyosarcoma, melanomas, and carcinomas) were observed to produce several growth regulatory factors. One class of factors competes for EGF binding and allows growth of normal rat fibroblasts in soft agar. These factors are designated transforming growth factors (TFGs). Certain of these human tumor cells also produce another class of factors which inhibits growth of human melanoma and carcinoma cells in soft agar and in monolayer cultures. These inhibitors of tumor cell growth have been designated tumor inhibiting factors (TIFs). Normal human and rat fibroblasts, however, are stimulated to proliferate by these same factors. Currently, extensive efforts are being directed towards the purification and characterization of this new class of factors produced by human tumor cells. Using commercially available crosslinking reagents, a cell surface receptor for a transforming growth factor was identified on normal rat fibroblasts. This putative TGF receptor was specifically competed for by unlabeled TGFs but not EGF.